The invention relates to pre-press, computerized color graphics, color reproduction, and electronic printing systems. In particular, the present invention relates to a method and apparatus for characterizing an output device that uses more than four colorants using technology for output devices that use at most four colorants.
The color gamut of a four-color printing process using CMYK is not very large and therefore some colors cannot be reproduced using only CMYK. Thus, processes using more than four inks have been developed in order to increase the color gamut. These additional inks are not “spot colors” used to create special effects, like luminescent inks. The additional inks are intrinsically part of the color separation process to create realistic images.
An example of printing with at least six inks is PANTONE's Hexachrome® system from PANTONE, Inc., Carlstadt, N.J., consisting of CMYK inks complemented with an Orange and a Green ink (CMYKOG). Another example is known as Küppers' ink set that uses CMYK, a Red, a Green, and a Blue ink (CMYKOGB). See H. Küppers: “Die Farbenlehre der Fernseh-, Foto- und Drucktechnik”, Du Mont Verlag., Köln, 1985.
Color management provides tools to match colors for color separation, color proofing, and color simulation. Specifically, color management provides tools to:                Convert images from one color space, e.g., that of a color scanner to another color space, e.g., that of the output device. This is called color separation.        Convert images that are for output on an output device to a proofer device such that colors match.        Simulate the appearance of images that are for output on an output on another device such as a color monitor.        
Color may be specified with only three color coordinates, e.g., CIE-XYZ or CIE Lab, so using six or seven or more colors for printing makes the calculation of the required colorant values, e.g., the separations for any particular color rather complex. There are more values to determine—6 or more—than the three coordinates defining the color, so the problem is “underdetermined,” i.e., there are more variables to be determined than inputs. The more inks, the more difficult the determining of the separations. Thus there is a need for a method for determining good color separations when there are a large number of inks used.
Known methods for determining good color separations are often based on first measuring the colorimetric properties of patches of different amounts of a set of inks printed in sequence on top of each other. Modem color management techniques for obtaining color separations, such as COLORSYNC™ (Apple Computer, Inc., Cupertino, Calif.) and the methods promoted by the International Color Consortium (ICC, see http://www.color.org) are based on this. The technologies use color profiles that convert the colorant values into some device independent color space such as CIE-XYZ or CIE-Lab, and also convert from the device independent space to the colorant values. While these techniques can produce accurate results, and also work for halftone images, they are practically limited to four ink systems. This is because of the large number of measurements of patches of combinations of more than 4 inks that would be needed. For example, the IT8.7.3 chart (American National Standards Institute [ANSI] Committee IT8 for Digital Data Exchange Standards) contains nearly a thousand patches for a four color output. Because of the large number of measurements, it is very difficult to characterize sets of more than four colorants, for example printing with six or seven colors. Furthermore, it is also difficult to make color separations for these printing systems based on their measured colorimetric properties. Thus, it is difficult to generate profiles that convert the more than four colorants, e.g., CMYKOG to CIE-XYZ or CIE-Lab—this needs lots of data to be measured and stored in the profile—and further to convert CIE-XYZ or CIE-Lab to the more than four colorants, e.g., CMYKOG. Thus there has been some effort to find better techniques applicable to more than four colorants or to different colorants.
Because color management technology using at most four colorants is well known, and because there is a body of expertise and software for manipulating images provided in CMYK, there is a desire for working with CMYK data that has the gamut of a process using more than four colorants.